Fix NPE from base64

java.lang.NullPointerException
    at org.apache.commons.codec.binary.Base64.encode(Base64.java:500)
    at org.apache.commons.codec.binary.Base64.encode(Base64.java:936)
    at org.candlepin.util.Util.toBase64(Util.java:250)
    at org.candlepin.servlet.filter.logging.LoggingRequestWrapper.getBody(LoggingRequestWrapper.java:65)
    at org.candlepin.servlet.filter.logging.LoggingFilter.logBody(LoggingFilter.java:114)
    at org.candlepin.servlet.filter.logging.LoggingFilter.logRequest(LoggingFilter.java:69)
    at org.candlepin.servlet.filter.logging.LoggingFilter.doFilter(LoggingFilter.java:54)
    at com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:163)
    at com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58)
    at org.candlepin.servlet.filter.CandlepinPersistFilter.doFilter(CandlepinPersistFilter.java:48)
    at com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:163)
    at com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58)
    at org.candlepin.servlet.filter.CandlepinScopeFilter.doFilter(CandlepinScopeFilter.java:57)
    at com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:163)
    at com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58)
    at com.google.inject.servlet.ManagedFilterPipeline.dispatch(ManagedFilterPipeline.java:118)
    at com.google.inject.servlet.GuiceFilter.doFilter(GuiceFilter.java:113)
    ...

The above would only happen if you have debug logging enabled AND using
the parallel product import. The hypothesis here is that Util.java is static,
and had an instance of Base64. The Base64 class has a buffer member variable
that isn't thread safe, so if multiple threads are calling Util.toBase64() at
the same time we run the risk of causing problems.